Packagers, and suppliers of adhesive products to packagers, have sought in various ways to provide, at the lowest cost, package resealing fastener means and methods which are reliable and easy for the consumer to use, which are contamination-tolerant and continue to provide effective resealing after continued reopenings, which do not require specialized label or tape application equipment, which can tolerate variations in the packaging material or substrate to which they are applied, which will operate satisfactorily over a reasonable spectrum of such materials with no need for customizing to each application, which do not require special or premium cost package substrate materials, and whose adhesive values are relatively insensitive to variations in time of seal and temperature of seal when the fastener means is originally applied, so that desired adhesion values are reliably and easily achieved and maintained.
In particular, the resealing fastener should have good differential adhesion. That is, with reference to the substrate material of a package on which the resealing fastener is provided, and on a uniform and reliable basis, the adhesion should be definitively higher at the permanent joint or interface between the fastener and the substrate material of the package than at the reseal joint or interface between the fastener and the same substrate material. There should also be high resistance to "pick," i.e., to partial transfer of the fastener adhesive to the substrate at the releasable interface, and such resistance should persist even after repeated closings and openings. These two characteristics, good differential adhesion and resistance to pick, have been achieved in prior art resealing systems only by sacrificing or compromising some of the other general objectives mentioned above.
Thus, the prior art has used thermally activated adhesives on thermoplastic films to effectively achieve very high adhesion or bonding at the permanent joint between fastener and substrate combined with a pressure-sensitive adhesive at the reseal side, but such systems require the costly use of special label application equipment. Minor variations of time of seal and temperature of seal when the fastener is originally applied greatly affect adhesion values. The protective liner which initially protects the reseal face must be more temperature-resistant and involves higher material costs. Adhesive ooze out of the sides of the fastener is also a practical problem.
The prior art has also used a carrier film or sheet to separate and prevent mixing of two pressure-sensitive adhesives of unlike adhesion. A "permanent" or high adhesion pressure-sensitive layer of adhesive interfaces with the substrate at the permanent joint, and a "removable" or relatively low adhesion layer of adhesive interfaces with the substrate at the reseal joint. One disadvantage is the cost of the carrier. Other cost disadvantages are the increased manufacturing costs associated with equipment to handle the carrier web and associated with increased production time where the construction is made as a two-pass operation. A major disadvantage is the need to customize to particular substrates. A progression of relative adhesions must be selected such that the adhesion at the permanent joint is highest, the adhesion at the reseal joint is lowest, and the differential between the adhesion of the "removable" adhesive to the carrier web and its adhesion to the substrate at the reseal joint is sufficiently high to avoid pick. These constraints not only often require customizing, but also tend to limit performance, since it may be necessary to compromise adhesion at the reseal joint in order to provide a reasonable differential from the adhesion at the interface between the "removable" adhesive and the carrier web.
Another expedient known in the prior art is simply to use a "removable" adhesive for reclosing. This gives no differential adhesion and "pick" is very high, so that transfer from one location on the substrate material to any other will readily occur.